The present invention relates to a weft insertion device for use on a band-gripper weaving machine. The weft insertion device comprises a driven, rotationally oscillating band wheel cooperating with a flexible insertion band having a gripper head and executing an oscillating movement into and out of the warp shed as it correspondingly unwinds from the winds onto the circumference of the band wheel. A guide means is provided for preventing the insertion band from separating from the outside circumference of the band wheel. The guide means is in the form of a cable of appropriate length which rests or presses against the outside of the insertion band. Both ends of the cable are anchored to the band wheel.
In a device of this type as disclosed in U.S. Pat. No. 4,274,449, the insertion band is wound around the circumference of the band wheel to the extent of about 300.degree.-330.degree. when the gripper head is in the position in which the head is fully and maximally withdrawn from the warp shed. The distance around the entire circumference of the band wheel is thus always a little greater than the maximum amplitude or excursion of the gripper head. The excursion is in turn directly related to the width of the machine.
Each time the direction of rotation of the band wheel is changed, the band wheel must be stopped and then accelerated again. In the charging process a moment of inertia of the mass of the band wheel must be overcome. Since the diameter and mass of the band wheel increase as the width of the weaving machine increases, the moment of inertia of the band wheel increases as well, thus limiting the speed of the weaving machine. Accordingly, for some time there have been ongoing efforts to substitute lighter structures for the band wheels which were originally in the form of cast wheels with spokes.
The lighter-construction wheels have a disc-like wheel body which is directly connected to the hub and has a honeycomb structure. The material used is either a light metal or a plastic. While these band wheels have generally had successful performance in practice, they are relatively expensive to manufacture. In addition there is the possibility that as weaving machines become substantially larger than those commonly in use today there may be problems related to the rigidity of present band wheels.
Accordingly, an object of the present invention is a refinement of a device of the type described initially above, whereby even with a very wide weaving machine the moment of inertia of the band wheels can be kept as small as possible. Also the rigidity of the band wheels may be maintained as high as possible without increasing the cost of manufacture.
These objects and others are achieved according to the present invention by a band wheel having a circumference which is smaller than the maximum excursion of the gripper head. Also, the insertion band is wound around the circumference of the band wheel to the extent of more than 360.degree. when the gripper head is in the position in which the head is fully and maximally withdrawn from the warp shed. Further, a cable rests or presses against the entire length of the insertion band which is wound onto the band wheel.
The invention deviates from the former approach of attempting to maintain high rotational speeds on wider machines by improving the design of the band wheels. Instead, it is proposed by the present invention to have an overlap in the winding of the insertion band around the circumference of the band wheel. By this technique, the diameter of the band wheel can be kept small, so that a wider machine will not entail either higher band wheel manufacturing costs or problems stemming from the inadequate rigidity of a larger diameter band wheel. The use of a band wheel with a lower moment of inertia also permits a higher rotational speed as well as a simplification and reduction in cost of the band wheel drive.
It should also be noted that with prior art devices wherewith the insertion band is wound around the band wheel to the extent of less than the total circumference of the wheel, the increased band wheel diameter accompanying increased machine width eventually leads to a condition where the necessary band wheel is too large for the machine. In other words, the machine is too short vertically to accommodate the band wheel. Also, an increased band wheel diameter itself adds to the overall width of the machine. Thus, the present invention affords significant dimensional advantages over the prior art. For example, the present invention limits the additional machine width occasioned by the band wheels when the basic machine width is increased. Further, the present invention permits the machine to be widened substantially without the machine having to be increased in height as well.